Wine Flavor and Tryptophan

Abstract

The intrinsic flavor properties of wine are dependent upon the flavor compounds’ composition and how these influence sensory perception. Aromatic amino acids are catabolized by the transamination of the amino group and the formation of alpha-keto acids, which are decarboxylated to the aldehydes. Furthermore, they create highly desired wine aroma by transforming into aromatic alcohol. While only traces of tryptophan metabolites could be determined in grapes and grape musts, their amounts increase significantly during fermentation. Even under optimal fermentation conditions, the most efficient thiol-releasing Saccharomyces cerevisiae wine strain realizes less than 5 % of the thiol-related flavor potential of grape juice. Tryptophanase is able to convert an odorless substrate, L-tryptophan, into the odorous products methyl mercaptan and indole. Tryptophanase-encoded cysteine-beta-lyase releases up to 25 times more 4-mercapto-4-methylpentan-2-one (4MMP) and 3-mercaptohexan-1-ol (3MH) and transforms these compounds into free thiols. In order to produce wines with more pronounced aromatic profiles, low-temperature alcoholic fermentations are utilized frequently. Tryptophan metabolism of yeast influences fermentation performance during low-temperature wine fermentation. Actually, tryptophan uptake by yeast cells is sensitive to decreases in membrane fluidity caused by either high pressure or low temperature. Thus, tryptophan permease-expressing yeast cells can grow up under low-temperature conditions. Moreover, nitrogen deficiency in grape musts is one of the main causes of ineffective wine fermentations. In this respect, ammonium is the preferred nitrogen source for biomass production. Indeed, ammonium supplementation has a greater impact on wine aroma and color intensity. Additionally, tryptophanase activity and the rate of tryptophan synthesis increase with ammonium. Tryptophan and its metabolites are considered to be potential precursors of an aroma compound, 2-aminoacetophenone (AAP). If the amount of AAP increases significantly during fermentation, “untypical aging off-flavor” (UTA) may occur. Hence, a significant correlation is found between the level of AAP concentrations and poor quality of wine. On the contrary, indol-3-ethanol which is mainly formed during alcoholic fermentation from tryptophan does not reveal unpleasant odor. Pressure treatments impart aged-like characteristics to the wines and can influence the red wine physicochemical and sensorial characteristics. L-tryptophan is of primary importance for cell growth under high-pressure conditions. Indeed, the availability of tryptophan is the key factor for the continuation of growth at high pressure. Considering the flavor profile of wine, the ethanol stress-tolerant yeast strains are highly desired by winemakers. The enhancement of expression of genes related to tryptophan biosynthesis increases the ethanol stress tolerance of yeast cells. The fruity odors of wine are largely derived from the synthesis of esters and higher alcohols during yeast fermentation. The adaptation of yeast to metabolic conditions throughout the fermentation process by secreting aromatic alcohols is known as quorum sensing. Quorum-sensing molecules, tryptophol and phenylethanol, regulate the morphogenesis of Saccharomyces cerevisiae during nitrogen starvation. The production of aromatic alcohols is autostimulated by tryptophol. Melatonin is detected in grape extract within the range of 120–160 ng/g, while its isomer is found in musts and finished wines. Melatonin levels are higher in red wine than in white wine. Although melatonin in wine, thus, is believed to be derived from grapes, yeast also produces up to 100 ng/mg protein melatonin. However, the contribution of melatonin to the flavor of wine is not well understood.